Re: [PATCH v3 3/3] mm/hmm/test: add self tests for HMM

[Ralph Campbell]

On 10/31/19 5:42 AM, Jason Gunthorpe wrote:
> On Wed, Oct 30, 2019 at 05:14:30PM -0700, Ralph Campbell wrote:
>
> > > Well, that is good, is it also under drivers/char? It kind feels like
> > > it should not be there...
> >
> > I think most of the test modules live in lib/ but I wasn't sure that
> > was the right place for the HMM test driver.
> > If you think that is better, I can easily move it.
>
> It would be good to get the various test people involved in this, I
> really don't know.

OK.
 
> > > > > It seems especially over-complicated to use a full page table layout
> > > > > for this, wouldn't something simple like an xarray be good enough for
> > > > > test purposes?
> > > >
> > > > Possibly. A page table is really just a lookup table from virtual address
> > > > to pfn/page. Part of the rationale was to mimic what a real device
> > > > might do.
> > >
> > > Well, but the details of the page table layout don't see really
> > > important to this testing, IMHO.
> >
> > One problem with XArray is that on 32-bit machines the value would
> > need to be u64 to hold a pfn which won't fit in a ULONG_MAX.
> > I guess we could make the driver 64-bit only.
>
> Why would a 32 bit machine need a 64 bit pfn?
>
> Jason

On x86, Physical Address Extension (PAE) uses a 64 bit PTE.
See arch/x86/include/asm/pgtable_32_types.h which includes
arch/x86/include/asm/pgtable-3level_types.h.